1. Field of the Invention
The present invention relates to a brake system in which, during normal operation, a wheel is braked by an electric braking force generation device; and when there is an abnormality in which the braking force generation device becomes inoperative, the wheel is braked with brake fluid pressure generated in a master cylinder upon a driver's braking operation.
2. Description of the Related Art
Japanese Patent Application Laid-open No. 2000-127805 discloses a so-called BBW (brake-by-wire) type brake system. In this system, a master cylinder generates brake fluid pressure upon depression of a brake pedal by a vehicle operator. During normal operation of the system, a power fluid pressure source for generating brake fluid pressure is operative, and a fluid pressure brake for braking a wheel is operated with brake fluid pressure generated by the power fluid pressure source when communication between the fluid pressure brake and the master cylinder is cut off by means of a master cylinder cut-off valve. When there is an abnormality in which the power fluid pressure source becomes inoperative, the master cylinder cut-off valve is opened to operate the fluid pressure brake with brake fluid pressure generated by the master cylinder. In addition, during the above-mentioned normal operation the brake fluid pressure generated by the master cylinder is absorbed by a stroke simulator to thus enable a stroke of the brake pedal.
In such a BBW type brake system, a depressing force cut-off valve (the master cylinder cut-off valve in the above publication) that provides or cuts off communication between the master cylinder and a wheel cylinder is a normally open solenoid valve. When there is an abnormality, such as a power source malfunction, and the solenoid is de-energized, the depressing force cut-off valve automatically opens, thus enabling the wheel cylinder to operate with a brake fluid pressure generated by the master cylinder. The solenoid of the depressing force cut-off valve is in an energized state during normal operation, which occupies most of the operation time apart from the above-mentioned abnormal operation. Thus, there is a demand for reduction in the power consumption of the solenoid.
The present invention has been accomplished under the above-mentioned circumstances, and it is an object thereof to reduce the power consumption of a solenoid of a depressing force cut-off valve in a BBW type brake system.